Aminoplast resins such as urea-formaldehyde resins and melamine-formaldehyde resins, also referred to as urea resins and melamine resins, are produced by reacting aqueous formaldehyde (formalin) with urea or melamine, respectively. The reaction, also known as a cook, involves methylolation and condensation, during which step methylolated molecules react with each other (crosslinking), forming larger molecules.
In resin manufacturing, condensation is normally stopped before all reactive sites have reacted and total crosslinking has taken place, and the resin is then transferred to the site of application where total irreversible crosslinking is induced in a specific application set-up by heat and optionally a catalyst.
A phenomenon that is especially known in melamine resins is that during the condensation reaction the so-called water tolerance of the resin decreases because larger melamine resin molecules exhibit a poor solubility in water. The water tolerance of a melamine resin is measured by adding water to a resin sample until the sample becomes turbid. Water tolerance is then defined as the amount of water that can be added to a unit-quantity of resin. Water tolerance is also a function of temperature. This phenomenon is used in a method known as the cloud point method to follow such reactions and make sure that the reaction is stopped before a certain critical or targeted point is reached. Continuing the reaction beyond this point will result in even larger molecules and consequently in too low a water tolerance level under given conditions and in precipitation of part of the melamine resin, an event that chemists normally want to avoid due to its negative consequences.
However, for certain applications it is requested to cross these limits because the progressing condensation and crosslinking give partially or totally crosslinked melamine resin droplets or particles with properties that are very suitable in such applications. As indicated above, crossing these limits of water tolerance will result in water-incompatible conglomerates in an aqueous phase, a situation that is very unpractical.
An elegant way to deal with this problem has been to create an emulsion where the water incompatible molecules are present in the aqueous water phase as droplets or particles, where the protective colloid is dispersing the hydrophobic phase in the aqueous phase. However, to be technically and economically attractive, such emulsions have to meet a range of criteria. Firstly, they have to be stable over a required period. Secondly, the droplet size or particle size of the partially or totally crosslinked melamine resin droplets or particles has to suit the specific application at hand. Thirdly, the economics of the total system have to be attractive, i.e. the raw material cost and the conversion cost have to be within certain competitive ranges. Finally, environmental issues must be considered.
U.S. Pat. No. 5,246,616 describes a process for producing particles of crosslinked melamine resin with a claimed uniform diameter starting from a water-soluble methyl-etherified melamine resin. Such a methyl-etherified melamine resin is condensed under stirring in the presence of a protective colloid such as a copolymer of acrylamide or methylacrylamide with acrylic acid or methylacrylic acid, a partial hydrolyzate of acrylamide and/or methyl-acrylamide, or a saponified product of acrylonitrile (typically a copolymer of methacrylic acid and acrylamide), at a pH of 5-8 (typically around 6) and at elevated temperatures (typically around 60.degree. C.). The solids content of the suspension is above 10 wt. % but below 30 wt. %, typically 13-27 wt. %. The resulting particle size is given as 4-12 .mu.m. The suspension is filtered and the separated filter cake dried and ground. The yield after separation and drying is quite poor, between 34-78 wt. %, depending on the particle size.
This process applies the emulsifying concept to prevent the precipitation of the melamine resin polymer, on becoming water insoluble during condensation (crosslinking). The precipitate will stick around the stirrer and on the reactor wall, thus preventing efficient stirring and presenting a process situation that is very difficult to handle on full scale.
However, a major disadvantage of this method is that it requires a partially methyl-etherified melamine resin instead of a melamine formaldehyde resin as starting material, thus adding to the product costs as well as representing a potential environmental problem due to the methanol involved. A minimum of 10% of the methylol groups must be etherified, and typically this is 35-50%, which is a considerable percentage.
Another disadvantage of this method is that the solids content of the resulting suspension is relatively low below 30% (typically 13-28%). In combination with the rather low yield after separation and filtration, this implies a high process volume to end-product volume ratio and also that a considerable amount of water be involved. As this water is likely to be contaminated with very fine emulsion particulates, this will pose a severe environmental problem as well as a cost issue.
Still another problem of this method concerns the control of the mean particle size diameter. Although the particle diameter is said to be uniform (within a given condition of reactor geometry and with a given formulation) the mechanism to adjust the desired particle size is not indicated and unclear. One way to influence the particle size seems to be the solids content in the resulting reaction liquid where a lower solids content seems to give a smaller particle size. This again adds to the low solids content problem as outlined above. Another way to influence particle size is said to be the addition of solvents like methanol and/or glycerine. Such additions are however very disadvantageous for environmental reasons and also add to the cost of the product.
In WO 97/07152 (PCT/US96/13384) a method is described for making cured (crosslinked) resin particles of melamine formaldehyde resin, by mixing an aqueous solution of the resin or its precursor with a water insoluble stabilizing agent, i.e. micro-crystalline cellulose. Sufficient water is provided to exceed the water tolerance level of the resin and an emulsion of resin droplets in the water phase. The resin molecules are then further cured (advanced) by heat resulting in partially or totally cured resin particles of uniform particle size as dispersed droplets or particles in an aqueous phase.
This method is a two- or three-step method. In step (1) a melamine formaldehyde resin is manufactured to a desired degree of partial cure (crosslinking), in step (2) a mixture of a water insoluble stabilizing agent in water is made, and then in step (3) this mixture is added to the melamine formaldehyde resin, forming an emulsion of resin droplets in an aqueous phase, after which the condensation or curing of the resin molecules in these droplets is advanced to the desired degree of cure (crosslinking). Two features of this method are said to be important: the use of a suitable stabilizing agent, preferably MCC (microcrystalline cellulose), and the time during the process at which the stabilizing agent water solution is added to the resin, or, in other words, the degree of partial cure of the melamine resin just before the stabilizing agent is added. This timing is claimed to control the size of the emulsified resin droplets or particles. The degree of curing must therefore be carefully followed by using differential scanning calorimetry (DSC), measuring the residual energy remaining in the resin, which indicates the remaining degree of cure, which is possible. The resin in step (1) is still totally water-soluble under the given conditions, but by adding water, the resin will pass the water tolerance limit and become partially water insoluble. Due to the presence of a stabilizing agent however, an emulsion is formed.
The choice of the stabilizing agent is said to be crucial as other stabilizing agents can cause foaming or wetting problems, while some also require the resin to be added to the colloid solution rather than vice versa, and result in non-uniformly sized resin particles. Microcrystalline cellulose (MCC) is the preferred agent, to be added in the range of 0.6% to 3.0% based on dry solids.
The major disadvantage of the above-described method is the fact that it is a three-step process with the need to prepare the aqueous mixture with the stabilizing agent, which has a volume equal to the resin solution, separately from this resin solution. This requires an additional mixing vessel or reactor besides the resin reactor and thus represents an extra investment cost.
Another disadvantage is the low solids content of the system, also related to the extra water volume, containing the stabilizing agent, that needs to be added to bring the melamine resin from a stage of still sufficiently water tolerant to a stage of insufficiently water tolerant, and to create an emulsion after mixing in the presence of the stabilizing agent.
All this extra water considerably reduces the solids content of the mixture, from an initial 50% to 25%. The total system thus becomes rather inefficient and uneconomical due to the large volumes of water that need to be handled, transported and processed or cleaned.